Patent GB-814114-A (GB)
Title: Radio communication system
IPC: H04B1/74
Filed: 1957-03-01
Abstract: 814,114. Radio receiving systems. STANDARD TELEPHONES & CABLES Ltd. March 1, 1957 [March 6, 1956], No. 6867/57. Class 40 (5). Radio communication system. General.-A radio communication system comprises a pair of transmitters having a common frequency and radiating the same signal, feeding cross-polarized antennµ, a co-operating pair of receivers with a common A.G.C. circuit, fed by cross-polarized antennae, the combined output of the receivers exhibiting a higher signal-to-noise ratio than either channel independently, since the in-phase signal components add linearly and the noise components in an R.M.S. fashion. System and embodiments.-A one-way system, Fig. 1, comprises a transmitting terminal 13, receiving terminal 15, and repeater 14, alarm circuits 10 and 24 which either indicate failure of either transmitter or failure of both transmitters, and alarm circuits 11 and 23 which monitor the signal condition at the respective receivers and disconnect a defective receiver from the common load. In a modification, Fig. 2, for both way communication employing separate frequencies F1 and F2 for " go " and " return " paths two transmitters and two receivers are used at the terminals, the crosspolarized antennµ being connected thereto through duplexers 27, 28, 41, 42. In a modification for space diversity reception the antennµ 31, 32, Fig. 3, at the terminal or repeater are spaced, in known manner, the difference in transmission path length being connected by insertion of artificial delay lines (not shown). Transmitters.-A pair of pulse transmitters together with their interconnection, Fig. 4, comprise known exciters 58 with synchronized crystal oscillators 60, monitors 59 and modulator reshapers 57. Monitors 59 each comprise a pickup loop and detector 77-78, an amplifier 79 to which the detected signal is applied, the amplified output being fed to a triode 81 which normally operates under zero bias conditions, because the positive D.C. voltage at its grid 80, due to detection of the negative pulse train output by diode 86, cancels the negative fixed bias, Under such conditions the valve current is sufficient to operate alarm relay 68, but should the pulse power output of amplifier 65 fall below a specified level, the current through triode 81 will fall and relay 68 will open. Receivers.-A pair of superheterodyne receivers 4 and 5, Fig. 5, each comprise known RF, IF and detector circuits 88-93 whose video output is applied to amplifier 94, and thence through transformer 95 to a common output load. Monitor 96 detects the presence of a video signal in the video amplifier 94, and controls the video output relay 97, disconnecting the output of a defective receiver in the absence of a video signal. In the event of complete failure in both receivers, the primary alarm, indicating signal failure, operates through alarm circuit 12, which is common to both receivers. Video amplifier.-The video amplifier 94 of receiver 4 comprises a stage 102 including an adjustable delay line 55, to correct phasing errors introduced by transmission line discrepancies when operating a space diversity system, feeding a three-stage amplifier 103, 104, 105, which applies signals to the common output 6. The A.G.C. control for receiver 4 is derived by a circuit including diode 107, capacitor 110 and potentiometer 108, which rectifies a portion of the output signal of the final video amplifier, an adjustable positive bias of smaller value than the peak pulse amplitude at the primary of the output transformer being applied, to the diode, the D.C. control voltage applied to IF amplifier 4 thus having a value proportional to the excess of pulse voltage over the bias voltage applied from voltage divider 108. The residual bias is cancelled by shunting capacitor 110 with diode 109, deriving a small bias voltage from resistor 111, through resistor 112 and applying it to the anode of diode 109, thereby effectively cancelling the residual noise bias. In order that the A.G.C. in the two receivers 4 and 5 will follow small fluctuations in signal level in one propaga. tion path relative to the other and produce alarm signals for significant differences, the circuits are coupled through conductors and switches 113-118, the alternative switching position coupling the A.G.C. circuits tightly for diversity reception. Receiver monitor.-The monitor circuit 96 compares the negative excursions of the output voltage with the positive excursions due to pulse signals, by sampling the receiver output derived from a small portion of the total cathode resistance of amplifier stage 105. Across diode 119 develops a negative D.C. voltage proportional to the positive pulse excursion of the output signal. Across diode 120 a negative voltage, proportional to the negative excursion of noise in the output signal, is developed. A differential relay 121 between the two diodes closes in the presence of a pulse and in the absense of noise, and opens in the presence of a predetermined noise level or under no-signal conditions. Relay 121 operates relay 97 to initiate the alarm circuitry 12.